Nowt too it really, you can make it as simple or as complicated as you want.
If you can wire a plug, the following isn't much harder
Average cost setup- 16amp socket in the engine bay then most fit a garage consumer unit (a mini version of your house fuse box) under R&R bed which then goes to appliances/sockets.
(16 amp socket)
Consumer Unit
Easy/Cheap method - You can do it on the really cheap with 16amp socket straight to inline RCD (like people use for lawnmowers) then 4way adapter or sockets
Inline RCD
The RCD and onsite unit (next to the site hookup) should kill the power before owt happens anyway if theirs a fault.
Or the other way buy a zig unit, such as the cf2000 if you want built in 12v leisure battery charger and separate fuses for lights etc.
If using reels, they need to be totally unwound, as they heat up if using heaters/big appliances! Easier to uncoil the 16amp lead and just chuck the cable under the van.
It is very DIY friendly, no need to get scared, Get it checked once fitted if your unsure.
Ive used all three methods in the past. A teardrop with just RCD to full blown t25 with zig unit, lights, leisure battery.
This time I'm just doing basic 240v hookup with RCD and no 12v as If I'm not on site I have coleman lamps and petrol stove anyway.